1. Field of the Invention
This invention relates to a method of forming an optically negative black-and-white silver image and, in particular, to a highly sensitive method of forming images using internal latent image type silver halide emulsions in which the grain surface is not intentionally fogged.
2. Description of the Prior Art
Formation of black-and-white images by means of a diffusion transfer process has long been applied to monochromatic instant photography. A representative method provides black-and-white positive silver images in the image-receiving layer of a film unit comprising a transparent support, an ordinary negative type silver halide emulsion layer coated thereon, and an image-receiving layer containing physical development nuclei (physical development centers) provided on a separate support, for example, paper, etc. After imagewise exposure, a developer containing a solvent for the silver halide such as a thiosulfate is expended between the silver halide emulsion layer and the image-receiving layer. The process can be dissected into three steps: (1) instantaneous development of a negative-type silver halide emulsion in the exposed areas to completion (insolubilization of the exposed areas), (2) dissolution and diffusion of the unexposed silver halide grains in the form of a soluble silver salt (dissolution of the unexposed areas), and (3) reduction of the dissolved silver salt in the image-receiving layer by the catalytic function of the physical development nuclei therein. The details of the process can be found in, for example, Photographic Silver Halide Diffusion Process by A. Rott and E. Weyde, 1972, The Focal Press, etc.
Likewise, photographic diffusion transfer processes providing negative black-and-white images are also known. For example, U.S. Pat. No. 3,615,438 and Photographic Science and Engineering, Vol. 15 (1), pp. 4-20 (1971) disclose a so-called solubilizing process which provides negative black-and-white images in the image-receiving layer by processing a typical nega-type silver halide emulsion with a processing formulation comprising a solvent for the silver halide and a heterocyclic compound containing a mercapto group whereby the degree of dissolution of the silver halide is appropriately controlled depending on the amount of exposure to produce a negative silver image. However, this solubilizing process suffers from a very narrow exposure latitude because the image tends to reverse at higher exposures. The exposure latitude is so narrow as to impose a severe restriction onto practical picture-taking conditions. Further, the presence of a development suppressing agent at a high concentration in the developer unacceptably lengthens the development period; and still another disadvantage is a keen dependence of the development results on the temperature during development.
Another process for obtaining negative black-and-white images based on diffusion transfer utilizes direct reversal type silver halide emulsions as is disclosed in U.S. Pat. No. 3,733,199. Such direct reversal type silver halide photographic emulsions include the so-called Herschel direct reversal type emulsions which are uniformly fogged prior to image exposure, and core-shell type direct reversal type emulsions in which the surface portion of each grain constituting the shell structure is fogged, as is disclosed in U.S. Pat. No. 3,367,778. Unfortunately, however, such direct reversal type silver halide emulsions have very low photographic speeds, which makes the above process very disadvantageous.